Photostimulable luminescent centre

Fig. 2. Schematic representation of the mechanism of photostimulated luminescence in BaBrF Eu. (A) Formation of a colour centre (F) under X-ray irradiation by trapping of an electron in a bromine vacancy with trapping of the hole formed in the valence band by a hole trapping centre (HT) in the vicinity (B) Release of the trapped electron by laser irradiation and transfer of the electron-hole recombination energy to Eu2+. Relaxations after electron transfers have not been represented.

Halides can form colour centres with absorption bands in the visible range. Doped with europium whose decay time of the allowed 5d —< 4f transition is of the order of 1 fis, they are the most appropriate materials. Because of the presence of fluorine, only BaCIF Eu and BaBrF Eu have a high enough chemical stability for screens fabrication. The highest efficiencies of photostimulated luminescence have been obtained with the fluorobromide (Amax = 390 nm). 

The mechanisms of the photostimulated luminescence of Eu2+-doped fluorohalides have been extensively studied. In BaBrF Eu an excess of fluorine favours the formation of F centres in bromine sites. This results in a red shift of the absorption induced by X-rays which increases the luminescence yield for stimulation by a He—Ne laser at 633 nm [69], Various interpretations have been proposed both about the nature of the hole-trapping centre and the electron-hole recombination mechanism [70], It was initially assumed that holes are trapped by Eu2+, leading to the formation of Eu,+ [71]. However after long X-ray irradiation no change in the EPR signal of Eu2+ was observed and the luminescence of Eu3+... 

It has been found that only the bromine F centers contribute to the photostimu-lability, although the X-ray irradiation creates both fluorine and bromine F centres [18]. These authors have also derived estimates of the concentrations of defect centers in a particular BaFBr Eu- sample. Even if these values arc not very reliable, they illustrate how complicated the physical mechanisms in a storage phosphor may be 82% of the centers created by irradiation are fluorine F centers or variants therebf these do not contribute to the photostimulable luminescence. The remaining 18% of the created centers are bromine F centers. Of these about one quarter arc spatially correlated to the hole center and the Eu ion, i.e. they yield PSL via a tunelling mechanism the others are not correlated and need thermal activation via the conduction band in order to yield PSL. These estimated concentrations depend strongly on the history of the sample and on the Eu concentration. 

Energy storage phosphors are materials in which an irradiation induces ionization of some ions followed by capture of the extracted electrons by vacancies, forming colour centres, or oxidizing cations. By irradiation in the absorption bands of the centres so created (photostimulation) or temperature rise, the trapped electrons can be released and the recombination energy with holes transferred to a luminescent ion. 

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